Magnetic random access memory (MRAM) devices are emerging as possible replacements for conventional RAM memory structures such as dynamic and static RAM structures. MRAM devices exhibit fast access speeds and greater immunity to radiation compared to conventional DRAM and SRAM structures, and do not require applied power to retain their logical state.
Thermally-assisted writing represents a conventional MRAM programming technique in which a heating current is supplied across the MRAM's barrier layer, the resistance of which causes the storage layer to heat to a predefined temperature. The free magnetic layer or the MRAM cell is preferably constructed from a material which exhibits a decreasing magnetic coercivity with increasing temperature, such that when the storage layer is sufficiently heated, lower magnitude writing currents can be used to re-orient the existing magnetic polarization of the storage layer.
Self-referencing is a technique also used in optimizing the performance of an MRAM device, and particularly, reading accuracy. In this technique, an MRAM cell undergoes operations in which the cell resistance is compared to resistances of the same or a similar cell when in a logical 1 and/or a logical 0 state. In a typical operation, the resistance corresponding to an initial state of the MRAM is stored, and subsequently a logical 1 is written to the cell and the resulting resistance stored. Similarly, a logical 0 may be written to the cell, and the result stored. The resistance corresponding to the MRAM's original state is subsequently compared against the resistance corresponding to the logical 1 or 0 self-reference measurements, and the logical state corresponding to the resistance having the closest correlation thereto is deemed to be the logical state of the MRAM cell.
As self-referencing operations include a programming phase in which logical 0 and/or 1 states are written to the MRAM cell, the aforementioned thermal-assist process is conventionally used to write the 0 and 1 logical states during this process. Thermally-assisted self-referencing presents difficulties, as it requires heating the MRAM cell to write either a 0 or 1 to the MRAM cell, waiting a predefined period to allow the cell to cool, and reading the cell. The process can be shortened if the MRAM cell is formed (e.g., with a heat sink) such that the cooling process occurs faster. In such an arrangement, however, the aforementioned thermally-assisted writing process is negatively impacted, as heating during the programming process will occur more slowly and require more energy in order to heat the MRAM cell to the required temperature.